The principle of self-cleaning coatings is well-known To achieve good self-cleaning of a surface, the surface not only has to be very hydrophobic, but also has to have some degree of roughness. A suitable combination of structure and hydrophobic properties permits even small amounts of water moving on the surface to entrain adherent dirt particles and clean the surface (see, for example, WO-A-96/04123; U.S. Pat. No. 3,354,022).
The prior art of EP-B-0 933 388 requires an aspect ratio>1 and a surface energy of less than 20 mN/m for these self-cleaning surfaces; the aspect ratio being defined here as the quotient which is the ratio of the height of the structure to its width. The abovementioned criteria are typically found in nature, for example, in the lotus leaf. The surface of the plant is composed of a hydrophobic waxy material and has elevations separated by a few μm. Water droplets substantially contact only the peaks of the elevations. There are many descriptions in the literature of water-repellent surfaces of this type.
Swiss patent 268 258 describes a process which generates structured surfaces by applying powders, such as, for example, kaolin, talc, clay, or silica gel. Oils and resins based on organosilicon compounds (Examples 1 to 5 of the Swiss patent) are used to secure the powders to the surface.
EP-A-0 909 747 teaches a process for generating a self-cleaning surface. The surface has hydrophobic elevations whose height is from 5 to 200 μm. A surface of this type is produced by applying a dispersion of pulverulent particles and of an inert material in a siloxane solution and then curing. The structure-forming particles are therefore secured to the surface by way of an auxiliary medium.
WO-A-00/58410 concludes that it is technically possible to render the surfaces of articles artificially self-cleaning. The surface structures necessary for this purpose, which are composed of elevations and depressions, have a separation in the range from 0.1 to 200 μm between the elevations of the surface structures, and have an elevation height in the range from 0.1 to 100 μm. The materials used for this purpose are composed of hydrophobic polymers or of lastingly hydrophobicized material. Release of the particles from the carrier matrix has to be prevented.
The use of hydrophobic materials, such as perfluorinated polymers, to produce hydrophobic surfaces is known. A further development of these surfaces consists in structuring the surfaces in the μm to nm range. U.S. Pat. No. 5,599,489 discloses a process in which a surface can be rendered particularly repellent by bombardment with particles of appropriate size, followed by perfluorination. H. Saito et al. in “Surface Coatings International”, 4, 1997, pp. 168 et seq., describe another process, in which particles composed of fluoropolymers are applied to metal surfaces, giving the surfaces thus generated markedly lower wettability with respect to water, with considerably reduced tendency toward icing.
This principle has been borrowed from nature. Small contact surfaces lower the level of vanderWaals interaction responsible for adhesion to flat surfaces with low surface energy. For example, the leaves of the lotus plant have elevations composed of a wax, and these reduce the area of contact with water.
Process for producing these structured surfaces are likewise known. Besides the use of a master structure to mold these structures in full detail by injection molding or embossing processes, there are also known processes that utilize the application of particles to a surface. This is disclosed, for example, in U.S. Pat. No. 5,599,489.
Recently, attempts have been made to provide self-cleaning surfaces on textiles. It has been found that self-cleaning surfaces can be generated by applying fine-particle SiO2 (Aerosils) to textiles. In this process, the Aerosils are bonded into the polymer matrix of the textile fiber, using a solvent
DE-A-101 18 348 describes polymer fibers with self-cleaning surfaces. In the prior art disclosure, the self-cleaning surface is obtained by exposure to a solvent, which comprises structure-forming particles, using the solvent to solvate the surface of the polymer fibers, adhesion of the structure-forming particles to the solvated surface, and removing the solvent The disadvantage of this process is that, during processing of the polymer fibers (spinning, knitting, etc.), the structure-forming particles, and therefore the structure that renders the surface self-cleaning, can become damaged or sometimes lost entirely, the result being that the self-cleaning effect is also lost.
DE-A-101 18 346 describes textile sheets with a self-cleaning and water-repellent surface composed of at least one synthetic and/or natural textile base material A, and of an artificial, at least to some extent, hydrophobic surface with elevations and depressions composed of particles that have been securely bonded to the base material A without adhesives, resins, or coatings. The hydrophobic surfaces are obtained by treating the base material A with at least one solvent that comprises the undissolved particles, and removing the solvent, whereupon at least some of the particles become securely bonded to the surface of the base material A. However, the disadvantage of this prior art process is the very complicated finishing of the textile surfaces. Moreover, this prior art process requires precise matching of the solvent to the base material of the textiles. However, in clothing there are generally mixed fabrics present, and this matching therefore becomes more complicated. If the matching of the solvents is not precise, the result can be irreparable damage to parts of the clothing. The textile surfaces therefore have to be treated prior to tailoring.
All of these coatings have the disadvantage that they are applied permanently to the articles and can therefore not be simply removed and reapplied in the event of impairment by scratching or discoloration or any other damage to the surface or surface structure. If this type of damage occurs, the article either has to be freed from the surface structure by a complicated method and retreated, or has to be disposed of.
WO-A-00/58410 describes a process for producing detachable coatings with dirt- and water-repellent properties. These coatings of the prior art are produced by spray-application of hydrophobic alcohols, such as nonacosan-10-ol, or of alkanediols, such as nonacosane-5,10-diol, or of waxes. The coatings of WO-A-00/58410 can be removed from articles by strong mechanical forces, e.g. scratching, brushing, or high-pressure water treatment, or by treatment with water which comprises detergents that disperse some of the structure-formers. A disadvantage of the prior art coatings disclosed in WO-A-00/58410 is the strong forces needed for mechanical removal of the coating. The use of strong forces for the mechanical removal of the coating runs the risk that when the coating is removed the article itself will also be damaged. Treatment with water that comprises detergents can likewise lead to damage to the article, depending on its nature.
DE-A-101 35 157 describes a process for the coating of textiles during a dry-cleaning procedure, in which structure-forming particles are added to the cleaning agent. The cleaning agents proposed comprise organic solvents that are relatively hazardous to ones health, e.g. trichloroethylene, and the use of the proposed cleaning agents leads to mechanical anchoring of the particles to the structure of the textiles. The result can be damage to the article to be coated.
It was therefore an object of the present invention to provide a process which can produce dirt- and water-repellent surface coatings on articles, and which can also treat articles composed of unstable materials to give a relatively stable coating, which, however, can be detached using simple means.